1. Technical Field
The present invention relates to counter-gravity casting apparatus for casting molten metal against gravity from a furnace into an above-situated casting mold.
2. Description Of Related Prior Art
Casting systems are known in which molten metal is delivered against gravity from a furnace into an above-situated casting mold for casting metal articles. Such systems are particularly useful for casting thin-sectioned articles as the metal is able to be delivered slowly and tranquilly under very low pressure (e.g., less than 10 psi) assuring development of the very thin sections of the casting.
Some of these systems deliver the metal by pressurizing the furnace with air or other gases to develop a differential pressure between the furnace and the mold, which differential pressure forces the metal from the furnace into the mold. Examples of such systems include those disclosed in the U.S. Pat. Nos. 3,842,893 to Booth, granted Oct. 22, 1974; 3,844,331 to Py et al, granted Oct. 29, 1974; 3,961,662 to Balevski et al, granted Jun. 8, 1976; 4,585,050 to Merrien et al, granted Apr. 29, 1986; 4,741,381 to Nishida et al, granted May 3, 1988 and 4,860,820 to Pereira, granted Aug. 29, 1989.
Such systems, however, are difficult to precisely control because the entire metal supply is under pressure. Any desired rapid changes in metal flow into the mold are countered by the momentum of the remaining pressurized supply.
Other systems are known to employ an electromagnetic pump in leu of air pressure for pumping the metal from the furnace into the mold. Examples of such systems are disclosed in the U.S. Pat. Nos. 4,213,494 to Carbonnel, granted Jul. 22, 1980; 4,714,102 to Koya, granted Dec. 22, 1987; and 4,967,827 to Campbell, granted Nov. 6, 1990. This type of system has an advantage over the pneumatic systems in that only the portion of the metal being pumped is under pressure, as opposed to the entire metal supply. As a result, rapid changes in metal flow are able to be made to the pumped metal and are not countered by the momentum of the remaining supply, as with the pneumatic system.
It is important to the making of high quality, defect-free castings that the casting conditions be precisely controlled. This includes both the cleanliness of the metal and also the manner in which it is delivered into the mold.
With most all of the systems known to employ an electromagnetic pump, the pump is accommodated in an open well of the casting furnace thereby exposing the metal in the furnace to the external atmosphere. This, of course, causes the metal to oxidize and, when casting aluminum, causes the aluminum to pick up or dissolve hydrogen into the melt. If such contaminated metal is delivered into the mold, the oxide and hydrogen impurities will form oxide inclusions and porosity defects within the resultant casting.
The open well also allows for a tremendous amount of heat to be lost from the melt. Supplying additional heat to the melt to compensate for the loss, however, is problematic in that it further adds to the contamination of the metal and further varies the metal viscosity. The additional contamination is attributable to aluminum's affinity for hydrogen increasing with increasing temperature so as to dissolve more hydrogen in solution as the metal temperature rises. The converse, however, is also true such that the hydrogen comes out of solution in the form of bubbles as the metal solidifies in the mold. The varying viscosity results from the temperature of the metal in the mold being nonuniform. Consequently, the characteristic output of the pump is disturbed, as its predictable output is dependent on consistent metal viscosity. Overheating the metal thus makes controlling the flow of metal into the mold much more difficult.
One system is known to employ a cover over the well to lessen heat loss and is disclosed in the aforementioned U.S. Pat. No. 4,967,827. The cover, however, does not protect the metal from contamination from the outside atmosphere as the environment within the space between the cover and the metal is not taught as being any different from that of the outside atmosphere. Thus, contamination of the metal still occurs as if there were no cover.
As mentioned, the other aspect to producing good castings is to precisely control the rate at which the metal is delivered into the mold. Filling the mold too fast leaves the very thin sections of the mold cavity unfilled, whereas filling too slowly produces porosity defects in the casting caused by uncontrolled solidification.
For each mold configuration, there exists an ideal manner in which the mold should be filled in order to produce the best possible casting. This can be expressed in terms of the ideal pressure of the pumped metal versus the casting cycle time. If the actual pumped metal pressure were controlled according to this idea schedule, then the best possible results would be achieved.
Systems known heretofore have been unable to control the metal flow according to such a schedule and thus produce less than the best possible castings. With one known system, the output of the pump is simply controlled as a matter of time. This system operates on the presumption that the output of the pump is characteristically related to the input voltage applied to the pump which, to a certain extent, it is. Such a control system, however, fails to take into consideration changing metal temperature (and viscosity), pump wear variations in characteristic outputs among different pumps, and variation in starting metal level among different casting cycles. All of these factors affect the relationship between the input voltage and associated output of the pump.
Another system is known to provide induction level sensors around the mold for detecting and monitoring the metal level throughout the casting cycle. This information is then used to make necessary corrections to the input voltage to the pump in order to conform the actual metal level with an ideal metal level versus casting cycle time schedule.
Such a control system, however, does not permit metal objects to be present in the mold during casting, such as is required when casting in place metal cylinder liners within a cylinder block of an internal combustion engine. Any metal objects present in the mold interfere with the operation of the sensors, making such a control system commercially impractical.
Another known system controls the fill by monitoring the temperature of the metal in different positions in the mold. It then adjusts pump output to conform actual conditions with an ideal metal temperature versus casting cycle time schedule. This system is disclosed in the aforementioned U.S. Pat. No. 4,213,494. Such a system, however, requires the mold to be fitted with numerous temperature sensors, which adds to the time, cost and complexity of making molds and castings.
Thus, the systems known heretofore are insufficient for controlling the operation of an electromagnetic pump in such a way so as to precisely counter-gravity fill a mold according to an ideal metal pressure versus casting cycle time schedule.